As a renewable energy resource, the use of solar cell modules is rapidly expanding. With increasingly complex solar cell modules, also referred to as photovoltaic modules, comes an increased demand for enhanced functional encapsulant materials. Photovoltaic (solar) cell modules are units that convert light energy into electrical energy. Typical or conventional construction of a solar cell module consists of at least 5 structural layers. The layers of a conventional solar cell module are constructed in the following order starting from the top, or incident layer (that is, the layer first contacted by light) and continuing to the backing (the layer furthest removed from the incident layer): (1) incident layer or front-sheet, (2) front-sheet (or first) encapsulant layer, (3) voltage-generating layer (or solar cell layer), (4) back-sheet (second) encapsulant layer, and (5) backing layer or back-sheet. The function of the incident layer is to provide a transparent protective window that will allow sunlight into the solar cell module. The incident layer is typically a glass plate or a thin polymeric film (such as a fluoropolymer or polyester film), but could conceivably be any material that is transparent to sunlight.
The encapsulant layers of solar cell modules are designed to encapsulate and protect the fragile voltage-generating layer. Generally, a solar cell module will incorporate at least two encapsulant layers sandwiched around the voltage-generating layer. The optical properties of the front-sheet encapsulant layer must be such that light can be effectively transmitted to the voltage-generating layer. Over the years, a wide variety of polymeric interlayers have been developed to be used as encapsulant layers. In general, these polymeric interlayers must possess a combination of characteristics including very high optical clarity, low haze, high impact resistance, shock absorbance, excellent ultraviolet (UV) light resistance, good long term thermal stability, excellent adhesion to glass and other solar cell laminate layers, low UV light transmittance, low moisture absorption, high moisture resistance, excellent long term weatherability, among other requirements. Widely used encapsulant materials utilized currently include complex, multicomponent compositions based on ethylene vinyl acetate (EVA), ionomer, polyvinylbutyral (PVB), thermoplastic polyurethane (TPU), polyvinylchloride (PVC), metallocene-catalyzed linear low density polyethylenes, polyolefin block elastomers, ethylene acrylate copolymers, such as poly(ethylene-co-methyl acrylate) and poly(ethylene-co-butyl acrylate), acid copolymers, silicone elastomers, epoxy resins, and the like.
EVA compositions, which have commonly been utilized as the encapsulant layer within solar cell modules, suffer the shortcomings of low adhesion to the other components incorporated within the solar cell module, low creep resistance during the lamination process and end-use and low weathering and light stability. These shortcomings have generally been overcome through the formulation of adhesion primers, peroxide curing agents, and thermal and UV stabilizer packages into the EVA compositions, which complicates the sheet production and ensuing lamination processes.
A more recent development has been the use of higher modulus ethylene copolymer ionomers within solar cell construction. For example, Maruyama et al. in Japanese Patent No. JP S56-116047, have disclosed a bi-layer encapsulant layer wherein the first layer is derived from EVA with a low VA content (20 wt % or lower), polyethylene, or soft PVC and the second layer derived from ionomer or EVA with high VA content (20 wt % or higher). Also exemplified is an ionomer/EVA bi-layer for use as an encapsulant layer, with the ionomer surface in contact with the outer solar cell layers, such as glass.
Okaniwa, et. al., in Japanese Patent No. JP H2-94574(A), have disclosed embossed films, such as PET film, to diffuse the incident light going into the solar cell. They further disclose “a tacky adhesive film layer” encapsulant, such as EVA, PVB, ionomer and polyethylene resins, with an encapsulant thickness of between 20 and 300 μm (0.8-11 mils).
Baum, Bernard, et. al., in “Solar Collectors. Final Report”, DOE/CS/35359-T1 (DE84011480), DOE6081.1, Contract No. AC4-78CS35359 (Springborn Laboratories, Inc.), June, 1983, have disclosed a list of usable solar cell encapsulant materials which include ethylene/acrylic acid, EAA-435® (a product of Dow Chemical Co.) and ionomer, Surlyn® 1707 (a product of E.I. du Pont de Nemours and Company).
U.S. Pat. Nos. 5,476,553; 5,478,402; and 5,733,382 have disclosed the use of encapsulant layers derived from ionomers formed by the partial neutralization of ethylene-methacrylic acid copolymers or ethylene-acrylic acid copolymers with inorganic bases and the use of Surlyn® 1601 and Surlyn® 1707 (E.I. du Pont de Nemours and Company). The laminates disclosed therein were produced with or without additional adhesion primers.
U.S. Pat. Nos. 5,741,370; 5,762,720 and 5,986,203 have disclosed a solar cell module back-sheet which is a thermoplastic polyolefin including a mixture of at least two ionomers.
U.S. Pat. Nos. 6,114,046 and 6,353,042, have disclosed a solar cell laminate encapsulant material which includes a layer of metallocene polyethylene disposed between two layers of ionomer.
U.S. Pat. No. 6,319,596 has disclosed a solar cell encapsulant layer comprising a polyolefin with an ionomer surface.
U.S. Pat. Nos. 6,320,116 and 6,586,271 have disclosed solar cell encapsulant layers with reduced creep through treatment with electron beam radiation.
U.S. Pat. No. 6,690,930 and US Patent Application No. 2003/0000568, have disclosed the use of Surlyn® 1705-1 and Surlyn® 1706 (E.I. du Pont de Nemours and Company) zinc ionomers as solar cell encapsulants.
However, there is a continuing need for solar cell encapsulant layers which provide adequate protection to the solar cell, have a long lifetime within the end-use and provide even greater levels of adhesion to the other solar cell laminate layers, preferably without the use of adhesion primers to simplify the producing processes. The present invention provides tailored solar cell encapsulant sheets meeting all these requirements.